Optical information recording medium, optical information recording/reproduction method, and optical information recording/reproduction apparatus

ABSTRACT

An optical information recording medium in which the ranges of the refractive index and thickness of a protective layer in a case where the protective layer is constituted by a plurality of layers are newly specified the refractive index and thickness of the protective layer are determined in these ranges, thereby ensuring that even if variation in thickness of the protective layer to ±3 μm is tolerated, spherical aberration in the optical information recording medium is substantially to within 30 mλrms.

This application is a divisional of U.S. patent application Ser. No.10/398,030, filed Aug. 19, 2003, which is a U.S. National PhaseApplication of PCT International Application PCT/JP01/08502 filed Sep.28, 2001.

TECHNICAL FIELD

The present invention relates to an optical information recording mediumwhich is irradiated with light from an optical head to perform recordingand/or reproduction of information, an optical informationrecording/reproduction method, and an optical informationrecording/reproduction apparatus.

BACKGROUND ART

Optical disks called “digital versatile disk (DVD)” have been put on themarket as a high-density high-capacity optical information recordingmedium. Such optical disks are now springing into wide use as recordingmediums for recording images, music and computer data. Optical diskshave a protective layer, which is considered specific to them, andtherefore have such characteristics as to be tough to scratch andcontamination. However, if the protective layer has a thickness error ora refractive index error, a third-order spherical aberration componentof wavefront aberration occurs, which can seriously affect informationrecording/reproduction characteristics.

An example of a conventional optical disk will be described withreference to drawings. FIG. 12 is a diagram schematically showing anoptical disk called a DVD. The optical disk 17 has a protective layer14, a recording layer 15, and a reinforcing substrate 16. Light having awavelength of 660 nm is converged by an objective lens 19, and therecording layer 15 is irradiated with this light on the protective layer14 side, thereby performing recording and/or reproduction ofinformation.

The objective lens 19 is designed so as to have a numerical aperture of0.6 and so that a third-order spherical aberration component ofwavefront aberration which occurs when light having a wavelength of 660nm passes through a light-transmitting flat plate having a refractiveindex of 1.58 and a thickness of 0.6 mm is substantially zero.

Polycarbonate is used for the protective layer 14, and a film containinga dielectric or a reflecting film is used for the recording layer 15.The reinforcing substrate 16 prevents the optical disk 17 from beingwarped or broken.

The protective layer 14 protects the recording layer 15 against air.Also, a surface 18 of the optical disk 17 is separated from therecording layer 15 by the protective layer 14 to prevent degradation ofrecording or reproduction performance due to dust on the surface 18 orscratches in the surface 18.

However, if the protective layer 14 has a thickness error or arefractive index error, a spherical aberration occurs in the spot formedon the recording layer 15 to badly affect recording/reproduction ofinformation. There is therefore a need to control the thickness andrefractive index of the protective layer 14.

FIG. 13 shows an example of specified values of the refractive index andthickness of the protective layer of a DVD. The abscissa represents therefractive index of the protective layer 14, and the ordinate representsthe thickness of the protective layer 14. The polygonal line in thegraph indicates a region of the refractive index and the thickness withwhich spherical aberration is within about 30 mλrms. For example, if thedesign values of the refractive index and thickness of the protectivelayer are fixed at a certain point on the dotted line in FIG. 13 andvariation in thickness is limited within the region, a disk capable ofnormal recording and reproduction of information can be obtained.

FIG. 14 shows a case where a sheet layer 11 and the reinforcingsubstrate 16 having the recording layer 15 are bonded together by anadhesive layer 13. The protective layer 14 is constituted by the sheetlayer 11 formed of polycarbonate or the like and the adhesive layer 13formed of an ultraviolet curing resin or the like. The refractive indexof the sheet layer 11 at a wavelength of 660 nm is 1.58 and therefractive index of the adhesive layer 13 at the same wavelength is1.51.

In such a case, spherical aberration occurs due to the differencebetween these refractive indices even if there is no error in thethickness 0.6 mm of the protective layer 14. For example, if thethickness of the sheet layer 11 is 0.56 mm and the thickness of theadhesive layer 13 is 40 μm, a spherical aberration of about 0.3 mλrmsoccurs, which is sufficiently small.

Thus, in the case where the numerical aperture is 0.6 and the wavelengthis 660 nm, the spherical aberration that occurs due to the differencebetween the refractive indices of the plurality of layers constitutingthe protective layer is sufficiently small and can therefore be ignored.That is, in the conventional art, the protective layer constituted by aplurality of layers can be treated as one layer and it is possible toavoid an adverse effect on recording and reproduction of information bycontrolling the refractive index and thickness of the protective layerwithin the certain ranges shown in FIG. 13 in accordance with a productstandard for an optical disk in which spherical aberration is limited.

In recent years, however, studies of next-generation optical diskshaving a higher recording density have been advanced in various regions.Next-generation optical disks are expected as a recording medium whichcan replace the currently dominant video tape recorders (VTRs), and thedevelopment of them is being promoted in a high pace.

As a means of increasing the recording density of optical disks, thereis a method of reducing the spot formed on the recording surface. Thisis achieved by increasing the numerical aperture for light from theoptical head and by reducing the wavelength.

This method, however, has the contrary effect of sharply increasing thespherical aberration due to a thickness error and a refractive indexerror of the protective layer. A need then rises to control thethickness and refractive index of the protective layer as in the case ofthe above-described DVD.

FIG. 15 is a diagram schematically showing an optical disk having anincreased recording density. The optical disk 27 has a protective layer24, a recording layer 25, and a reinforcing substrate 26. Light having awavelength of 400 to 410 nm is converged by an objective lens 29, andthe recording layer 25 is irradiated with this light on the protectivelayer 24 side, thereby performing recording and/or reproduction ofinformation.

The numerical aperture of the objective lens 29 is large, about 0.85.Therefore two lenses are used as the objective lens 29. The objectivelens 29 is designed so that a third-order spherical aberration componentof wavefront aberration which occurs when light having a wavelength of405 nm passes through a light-transmitting flat plate made ofpolycarbonate or the like and having a refractive index of 1.62 and athickness of 100 μm is substantially zero. The spot formed on therecording layer 25 is reduced in size by increasing the numericalaperture and by reducing the wavelength to achieve an increase indensity.

As the recording layer 25, a film containing a dielectric or areflecting film is used. The reinforcing substrate 26 prevents theoptical disk 27 from being warped or broken.

The protective layer 24 protects the recording layer 25 against air.Also, a surface 28 of the optical disk 27 is separated from therecording layer 25 by the protective layer 24 to prevent degradation ofrecording or reproduction performance due to dust on the surface 28 orscratches in the surface 28.

A sheet layer 21 and the reinforcing substrate 26 having the recordinglayer 25 are bonded together by an adhesive layer 23. The protectivelayer 24 is thus formed of two layers.

The sheet layer 21 is formed of polycarbonate or the like and theadhesive layer 23 is formed of an ultraviolet curing resin or the like.The refractive index of the sheet layer 21 at a wavelength of 405 nm is1.62 and the refractive index of the adhesive layer 23 at the samewavelength is 1.53. In such a case, spherical aberration occurs due tothe difference between these refractive indices even if there is noerror in the design thickness 100 μm of the protective layer 24.

For example, even if the thickness of the sheet layer is 60 μm; thethickness of the adhesive layer 23 is 40 μm; and the thickness of theprotective layer 24 is 100 μm, a spherical aberration of 5.3 mλrmsoccurs. This spherical aberration remains initially as residualaberration.

Apart from this, a spherical aberration due to variation in thicknesswhich occurs in manufacture of the optical disk is also added.Ordinarily, a thickness variation of about 3 μm occurs and a sphericalaberration of 30 mλrms due to the thickness variation results.

Consequently, even if the thickness of the protective layer 24 can beadjusted to 100 μm, the total spherical aberration including theabove-mentioned residual spherical aberration 5.3 mλrms is 35.3 mλrmsand normal recording or reproduction cannot be performed.

FIG. 16 shows comparison between spherical aberration due to theadhesive layer when the numerical aperture is 0.6 and the wavelength is660 nm (DVD) and spherical aberration due to the adhesive layer when thenumerical aperture is 0.85 and the wavelength is 405 nm. As can beunderstood from this graph, a large spherical aberration is caused dueto the different refractive indices of the plurality of layersconstituting the protective layer when the numerical aperture is largeand the wavelength is short. This aberration is 15 times or more largerthan that in the case of the DVD. If a variation in thickness of theprotective layer is further added, the acceptable limit sphericalaberration is exceeded, resulting in failure to perform normal recordingor reproduction.

DISCLOSURE OF THE INVENTION

In view of the above-described problem of the conventional optical disk,an object of the present invention is to provide an optical informationrecording medium, an optical information recording/reproduction method,and an optical information recording/reproduction apparatus in whichspherical aberration of the protective layer constituted by a pluralityof layers is reduced in comparison with that in the conventional art toenable normal recording and/or reproduction.

A first aspect of the present invention is an optical informationrecording medium comprising a recording layer, and a protective layerincluding at least first to mth layers (m≧2),

-   -   wherein when i is an integer satisfying 1≦i≦m; a refractive        index of the ith layer in the protective layer at a wavelength        of 405 nm is n_(i); and a thickness of the ith layer is d_(i),        and when (a) a combined refractive index n of one layer which is        substantially equivalent to the plurality of layers constituting        the protective layer and substituted for the plurality of layers        is specified as n ((n₁d₁+n₂d₂+ . . . n_(m)d_(m))/(d₁/n₁+d₂/n₂+ .        . . +d_(m)/n_(m)))^(0.5); (b) a combined thickness d of said one        layer is specified as d=((n₁d₁+n₂d₂+ . . .        n_(m)d_(m))×(d₁/n₁+d₂/n₂+ . . . +d_(m)/n_(m)))^(0.5); and (c) an        expression f(n): f(n) =−109.8n³+577.2n²−985.5n+648.6 is        specified as a design criterion relating to the combined        thickness of the protective layer,    -   the refractive index n_(i) of the ith layer and the combined        refractive index n is equal to or larger than 1.45 and equal to        or smaller than 1.65, and a difference d−f(n) between the        combined thickness and the design criterion is equal to or        larger than −10 μm and equal to or smaller than 10 μm.

A second aspect of the present invention is the optical informationrecording medium according to the first aspect of the present invention,wherein the value of d−f(n) is equal to or larger than −3 μm and equalto or smaller than 3 μm.

A third aspect of the present invention is an optical informationrecording medium comprising at least first and second recording layers,and a protective layer including a plurality of layers,

-   -   wherein when first to kth layers (k≧2) in the plurality of        layers in the protective layer between a surface of the optical        information recording medium and the first recording layer are        specified as a first protective layer, and first to mth layers        (m>k) in the plurality of layers in the protective layer between        the surface of the optical information recording medium and the        second recording layer are specified as a second protective        layer,    -   when i is an integer satisfying 1≦i≦m; the refractive index of        the ith layer in the protective layer at a wavelength of 405 nm        is n_(i); and the thickness of the ith layer is d_(i),    -   when (a−1) a combined refractive index n of one layer which is        substantially equivalent to the first protective layer and        substituted for the first protective layer is specified as        n=((n₁d₁+n₂d₂+ . . . +n_(k)d_(k))/(d₁/n_(l)+d₂/n₂+ . . .        +d_(k)/n_(k)))^(0.5); (b−1) a combined thickness d of said one        layer is specified as d=((n₁d₁+n₂d₂+ . . .        +n_(k)d_(k))×(d₁/n₁+d₂/n₂+ . . . d_(k)/n_(k)))^(0.5); and (c−1)        an expression f(n): f(n)=−105.8n³+551.5n²−936.9n+605.2 is        specified as a design criterion relating to the thickness of the        first protective layers; and    -   when (a-2) a combined refractive index n of one layer which is        substantially equivalent to the second protective layer and        substituted for the second protective layer is specified as        n=((n₁d₁+n₂d₂+ . . . +n_(m)d_(m))/(d₁/n₁+d₂/n₂+ . . .        +d_(m)/n_(m)))^(0.5); (b-2) a combined thickness d of said one        layer is specified as d=((n₁d₁+n₂d₂+ . . .        +n_(m)d_(m))×(d₁/n₁+d₂/n₂+ . . . +d_(m)/n_(m)))^(0.5); and (c-2)        an expression g(n): g(n)=−138.7n³+723.7n²−1228.7n+796.0 is        specified as a design criterion relating to the thickness of the        second protective layers,    -   the refractive index n_(i) of the ith layer and the combined        refractive index n of the first and second protective layers is        equal to or larger than 1.45 and equal to or smaller than 1.65,        a difference d−f(n) between the combined thickness d of the        first protective layer and the design criterion expression f(n)        is equal to or larger than −10 μm, and a difference d−g(n)        between the combined thickness d of the second protective layer        and the design criterion expression g(n) is equal to or smaller        than 10 μm.

A fourth aspect of the present invention is an optical informationrecording medium comprising a recording layer, and a protective layerincluding at least two layers,

-   -   wherein when a refractive index of a particular one of the        layers in the protective layer at a wavelength of 405 nm is n₁;        a thickness of the particular one of the layers is d₁ (μm); and        a thickness of the layers in the protective layer other than the        particular one layer is d₂ (μm),    -   when (a) equations A, B, C, and D including said d₂ are        specified as A=1.280d₂−109.8, B=−6.652d_(2+577.2),        C=11.27d₂−985.5, and D=−6.257d_(2+648.6), and    -   (b) an expression f(n₁): f(n₁)=A·n₁ ³+B·n₁ ²+C·n₁+D is specified        as a design criterion relating to the thickness of the        protective layer,    -   the refractive index n_(i) is equal to or larger than 1.45 and        equal to or smaller than 1.65, a refractive index of the layers        in the protective layer other than said particular one layer at        a wavelength of 405 nm is equal to or larger than 1.50 and equal        to or smaller than 1.55, and a value of d₁+d₂−f(n₁) is equal to        or larger than −10 μm and equal to or smaller than 10 μm.

A fifth aspect of the present invention is the optical informationrecording medium according to the fourth aspect of the presentinvention, wherein the value of d₁+d₂−f(n₁) is equal to or larger than−3 μm and equal to or smaller than 3 μm.

A sixth aspect of the present invention is an optical informationrecording medium comprising a recording layer, and a protective layerincluding at least two layers,

-   -   wherein when a thickness of a particular one of the layers in        the protective layer is d₁ (μm); a thickness of the layers in        the protective layer other than said particular one layer is d₂        (μm); and an equation f(d₁) including said d₁ is specified as        f(d₁)=−0.986d₁+98.6,    -   a refractive index of said particular one layer at a wavelength        of 405 nm is equal to or larger than 1.61 and equal to or        smaller than 1.63, a refractive index of the layers in the        protective layer other than said particular one layer at the        wavelength 405 nm is equal to or larger than 1.50 and equal to        or smaller than 1.55, and a value of d₂−f(d₁) is equal to or        larger than −10 μm and equal to or smaller than 10 μm.

A seventh aspect of the present invention is the optical informationrecording medium according to the sixth aspect of the present invention,wherein the value of d₂−f(d₁) is equal to or larger than −3 μm and equalto or smaller than 3 μm.

An eighth aspect of the present invention is an optical informationrecording medium comprising a recording layer, and a protective layerincluding at least two layers,

-   -   wherein when a thickness of a particular one of the layers in        the protective layer is d₁ (μm); a thickness of the layers in        the protective layer other than said particular one layer is d₂        (μm); and an equation f(d₁) including said d is specified as        f(d₁)=−1.002d₁+98.6,    -   a refractive index of said particular one layer at a wavelength        of 405 nm is equal to or larger than 1.49 and equal to or        smaller than 1.51, a refractive index of the layers in the        protective layer other than said particular one layer at the        wavelength 405 nm is equal to or larger than 1.50 and equal to        or smaller than 1.55, and a value of d₂−f(d₁) is equal to or        larger than −10 μm and equal to or smaller than 10 μm.

A ninth aspect of the present invention is the optical informationrecording medium according to the eighth aspect of the presentinvention, wherein the value of d₂−f(d₁) is equal to or larger than −3μm and equal to or smaller than 3 μm.

A tenth aspect of the present invention is an optical informationrecording medium comprising a recording layer, and a protective layerincluding at least two layers,

-   -   wherein when a thickness of a particular one of the layers in        the protective layer is d₁ (μm); a thickness of the layers in        the protective layer other than said particular one layer is d₂        (μm); and an equation f(d₁) including said d₁ is specified as        f(d₁)=−d₁+98.6,    -   a refractive index of said particular one layer at a wavelength        of 405 nm is equal to or larger than 1.52 and equal to or        smaller than 1.54, a refractive index of the layers in the        protective layer other than said particular one layer at the        wavelength 405 nm is equal to or larger than 1.50 and equal to        or smaller than 1.55, and a value of d₂−f(d₁) is equal to or        larger than −10 μm and equal to or smaller than 10 μm.

An eleventh aspect of the present invention is the optical informationrecording medium according to the tenth aspect of the present invention,wherein the value of d₂−f(d₁) is equal to or larger than −3 μm and equalto or smaller than 3 μm.

A twelfth aspect of the present invention is an optical informationrecording medium comprising a recording layer, and a protective layerincluding at least first to mth layers (m≧2),

-   -   wherein when i is an integer satisfying 1≦i≦m; a refractive        index of the ith layer in the protective layer at a        predetermined wavelength is n_(i); and a thickness of the ith        layer is d_(i), and when (a) a combined refractive index n of        one layer which is substantially equivalent to the plurality of        layers constituting the protective layer and substituted for the        plurality of layers is specified as n=((n₁d₁+n₂d₂+ . . .        +n_(m)d_(m))/(d₁/n₁+d₂/n₂+ . . . +d_(m/n) _(m)))^(0.5); (b) a        combined thickness d of said one layer is specified as        d=((n₁d₁+n₂d₂+ . . . +n_(m)d_(m))×(d₁/n₁+d₂/n₂+ . . .        d_(m)/n_(m)))^(0.5); and (c) an expression f(n) which has the        combined refractive index n as a variable, and which is one of,    -   f(n)=−109.8n³+577.2n²−985.5n+648.6,    -   f(n)=−105.8n³+551.5n²−936.9n+605.2, and    -   f(n)=−138.7n³+723.7n²−1228.7n+796.0        is specified as a design criterion relating to the combined        thickness of the optical information recording medium,    -   a difference between the combined thickness d and the design        criterion expression f(n) is equal to or larger than −10 μm and        equal to or, smaller than 10 μm.

A fifteenth aspect of the present invention is an optical informationrecording/reproduction method comprising performing at least one ofrecording and reproduction of information on the optical informationrecording medium according to the first, the fourth, the sixth, theeighth, the tenth, or the twelfth aspect of the present invention byusing an optical head having aberration correction means of correctingaberration which occurs due to the thickness of the protective layer ofthe optical information recording medium.

A sixteenth aspect of the present invention is an optical informationrecording/reproduction apparatus comprising:

-   -   the optical information recording medium according to any one of        the first to the twelfth aspect of the present inventions;    -   an optical head;    -   a rotating unit which causes rotation of said optical        information recording medium;    -   a control unit which controls said optical head; and    -   recording/reproduction means of performing at least one of        recording of information on said optical information recording        medium and reproduction of information from said optical        information recording medium.

A seventeenth aspect is the optical information recording/reproductionapparatus according to the sixteenth aspect of the present invention,wherein said optical head has aberration correction means of correctingaberration which occurs due to a thickness of the protective layer ofsaid optical information recording medium.

Thus, occurrence of spherical aberration is limited to enable optimizedrecording and reproduction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing the construction of an opticaldisk in Embodiment 1 of the present invention;

FIG. 2 is a diagram showing the relationship between combined refractiveindex n and combined thickness d in Embodiment 1 of the presentinvention;

FIG. 3 is a diagram showing the relationship between combined refractiveindex n and combined thickness d in Embodiment 2 of the presentinvention;

FIG. 4 is a diagram showing the relationship between combined refractiveindex n₁ and thickness d₁+d₂ in Embodiment 3 of the present invention;

FIG. 5 is a diagram showing the relationship between combined refractiveindex n₁ and thickness d₁+d₂ in Embodiment 3 of the present invention;

FIG. 6 is a diagram showing the relationship between thickness d₁ andthickness d₂ in Embodiment 4 of the present invention;

FIG. 7 is a diagram showing the relationship between thickness d₁ andthickness d₂ in Embodiment 5 of the present invention;

FIG. 8 is a diagram showing the relationship between thickness d₁ andthickness d₂ in Embodiment 6 of the present invention;

FIG. 9 is a cross-sectional view showing an example of an optical headincorporating a spherical aberration correction element;

FIG. 10 is a diagram showing recording on or reproduction from anoptical disk with an optical head incorporating a spherical aberrationcorrection element;

FIG. 11 is a diagram showing an optical informationrecording/reproduction apparatus of the present invention;

FIG. 12 is a cross-sectional view showing the construction of aconventional optical disk;

FIG. 13 is a diagram showing the relationship between the refractiveindex and the thickness of the conventional optical disk;

FIG. 14 is a cross-sectional view showing the construction of aconventional optical disk;

FIG. 15 is a cross-sectional view showing the construction of ahigh-density optical disk;

FIG. 16 is a diagram showing the relationship between the thickness ofan adhesive layer and spherical aberration;

FIGS. 17(a) and 17(b) are diagrams for explaining derivation ofequations 1 and 2 in Embodiment 1 of the present invention; and

FIG. 18 is a cross-sectional view showing the construction of an opticaldisk in Embodiment 2 of the present invention.

DESCRIPTION OF SYMBOLS

-   1, 11, 21 Sheet layer-   2 Coating layer-   3, 13, 23 Adhesive layer-   4, 14, 14 Protective layer-   5, 15, 25 Recording layer-   6, 16, 26 Reinforcing substrate-   7, 17, 27 Optical disk-   8, 18, 28 Surface-   9, 19, 29 Objective lens-   10 Spherical aberration correction element-   30 Semiconductor laser-   31 Prism-   32 Condenser lens-   33 Mirror-   34 Spherical aberration correction element-   35 Objective lens-   36 Objective lens driver-   37 Optical disk-   38 Cylindrical lens-   39 Photodetector-   40 Optical head-   41 Objective lens-   42 Motor-   43 Shaft-   44 Head control circuit-   45 Signal processing circuit.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described with reference tothe drawings.

(Embodiment 1)

FIG. 1 is a cross-sectional view of an optical disk which is anembodiment of the optical information recording medium of the presentinvention.

Referring to FIG. 1, an optical disk 7 has a protective layer 4, arecording layer 5, and a reinforcing substrate 6. Light having awavelength of 400 to 410 nm is converged by an objective lens 9, and therecording layer 5 is irradiated with this light on the protective layer4 side, thereby performing recording and/or reproduction of information.

The numerical aperture of the objective lens 9 is large, about 0.85.Therefore two lenses are used as the objective lens 9. The objectivelens 9 is designed so that a third-order spherical aberration componentof wavefront aberration which occurs when light having a wavelength of405 nm passes through a light-transmitting flat plate made ofpolycarbonate or the like and having a refractive index of 1.62 and athickness of 100 μm is substantially zero.

As the recording layer 5, a film containing a dielectric or a reflectingfilm is used. The reinforcing substrate 6 prevents the optical disk 7from being warped or broken. The protective layer 4 protects therecording layer 5 against air. Also, a surface 8 of the optical disk 7is separated from the recording layer 5 by the protective layer 4 toprevent degradation of recording or reproduction performance due to duston the surface 8 or scratches in the surface 8.

In the optical disk 7, a sheet layer 1 and the reinforcing substrate 6having the recording layer 5 are bonded together by an adhesive layer 3,and a coating layer 2 is formed as a surface coating for protectionagainst scratch. The protective layer 4 is thus formed of three layers.

Polycarbonate or the like is used for the sheet layer 1, an acrylicresin or the like is used for the coating layer 2, and an ultravioletcuring resin or the like is used for the adhesive layer 3. Therefractive index of the sheet layer 1 at a wavelength of 405 nm is 1.62,the refractive index of the coating layer 2 at the same wavelength is1.50, and the refractive index of the adhesive layer 3 at the samewavelength is 1.53. Spherical aberration occurs due to the differencesbetween these refractive indices even if there is no error in the designthickness 100 μm of the protective layer 4. Because of the largenumerical aperture and the short wavelength, the amount of thisspherical aberration is large and not negligible.

In the optical disk of the present invention, if the refractive indicesof the sheet layer 1, the coating layer 2 and the adhesive layer 3 at awavelength of 405 nm are n₁, n₂, and n₃, respectively; and the thicknessof these layers are d₁, d₂, and d₃ (μm), 1.45≦n₁≦1.65, 1.45≦n₂≦1.65,1.45≦n₃≦1.65, 1.45≦n≦1.65, and −3 μm≦d−f(n)≦3 μm are satisfied.

The inventor of the present invention newly introduced the concept of acombined refractive index and a combined thickness, i.e., the refractiveindex and thickness of one layer which is equivalent to a plurality oflayers constituting a protective layer, and which can be substituted forthese layers, and thereby enabled fabrication of an optical disk inwhich spherical aberration was reduced.

In this embodiment, the combined refractive index n can be shown by thefollowing equation 1.

(Equation 1)n=((n ₁ d ₁ +n ₂ d ₂ +n ₃ d ₃)/(d ₁ /n ₁ +d ₂ /n ₂ +d ₃ n ₃))^(0.5)

The combined thickness d can be expressed by the following equation 2.

(Equation 2)d=((n ₁ d ₁ +n ₂ d ₂ +n ₃ d ₃)×(d ₁ /n ₁ +d ₂ /n ₂ +d ₃ n ₃))^(0.5)

As mentioned above, these value n and d represent a combined refractiveindex and a combined thickness obtained by replacing the plurality oflayers with one layer equivalent to the plurality of layers. Aberrationof light passed through the plurality of layers constituting theprotective layer and aberration of light passed through the oneequivalent layer derived as described above are substantially equal toeach other.

A description will be given below of derivation of these equations 1 and2.

An expression f(n) shown in the equation 3 below is a third-orderapproximate curve passing through discrete points at which sphericalaberration is substantially zero in aberration calculation based on raytracing. The value of f(n) corresponds to the combined thickness d.

(Equation 3)f(n)=−109.8n ³+577.2n ² −985.5n+648.6

The thickness of the protective layer is specified within the range of±3 μm from the value indicated by this curve f(n).

The combined refractive index n and the combined thickness d satisfyingthis condition are in the region surrounded by curves and straight linesin FIG. 2. The dotted line is the curve f(n) along which the sphericalaberration is substantially zero, i.e., a design criterion for thecombined thickness of the protective layer 4.

If the combined refractive index n and the combined thickness d obtainedfrom the thicknesses and the refractive indices of the plurality oflayers constituting the protective layer are determined within thisregion (region surrounded by the solid lines in FIG. 2) by theabove-described equations 1 and 2, an optical disk can be fabricated inwhich the spherical aberration is substantially within 30 mλrms.

For example, if a disk design is such that n₁=1.62, d₁=50.0 μm, n₂=1.50,d₂=5.0 μm, n₃=1.53, and d₃=44.2 μm, then a combined refractive indexn=1.57 and a combined thickness d=99.2 μm are obtained by equations 1and 2. A point which is determined by the combined refractive index andcombined thickness obtained in this manner becomes a point Z on thecurve f(n) shown in FIG. 2, such that the initial residual sphericalaberration is substantially zero. In this case, even if variation inthickness to ±3 μm is tolerated, an optical disk can be fabricated inwhich the spherical aberration is limited substantially to 30 mλrms.

Thus, an optical disk in accordance with this embodiment of the presentinvention is designed by considering the thicknesses and refractiveindices of the plurality of layers constituting the protective layer,and can therefore be fabricated so that occurrence of sphericalaberration is limited.

The meaning of consideration of the thicknesses and refractive indicesof the plurality of layers constituting the protective layer will befurther described.

That is, the combined refractive index n and the combined thickness dsatisfying equation 3 are distributed on the dotted line shown in FIG.2, as described above. Therefore theoretically, there are an infinitenumber of combinations of the combined refractive index and the combinedthickness satisfying equation 3, for example. Also, theoretically, thereare many combinations of the refractive index n_(i) and the thicknessd_(i) (i=1, 2, 3) of each layer satisfying the relation betweenequations 1 and 2.

If only a suitable realizable combination of the refractive index n_(i)and the thickness d_(i) of each layer is selected from many candidatecombinations satisfying these equations, the initial residual sphericalaberration is substantially zero. Accordingly, with respect tofabrication of an optical disk in which the overall spherical aberrationis substantially within 30 mλrms, variation to ±3 μm may be tolerated inthe thickness of the protective layer.

Thus, according to the present invention, an optical disk based on anovel design technique is provided unlike the conventional optical diskthat has been fabricated from the viewpoint of considering onlyvariation in thickness of the protective layer and minimizing thevariation in order to reduce the spherical aberration.

That is, according to the present invention, a combination of therefractive index n_(i) and thickness d_(i) of each layer is selectedfrom many candidate combinations satisfying predetermined equations(equations 1 to 3 in this embodiment) to enable tolerance of ±3 μm,which is a limit value of variation in thickness of the protective layerin a current fabrication technique.

Also, the range of selection of various parameters including therefractive index n_(i) and thickness d_(i) of each layer is extended toincrease the degree of design freedom.

Derivation of the above-described equations will be described withreference to FIGS. 17(a) and 17(b).

FIG. 17(a) shows the state of refraction of light passing through aplurality of layers, and FIG. 17(b) shows the state of refraction oflight passing through one layer. To equalize an aberration of lightpassing through the plurality of layers and an aberration of lightpassing through one layer, a condition equation a expressing theequality of the optical path lengths and a condition equation bexpressing the equality of the positions of emergent light aresimultaneously satisfied with respect to rays incident at an angle of φ.

(Condition Equation a)n ₁ d ₁/cosθ₁ +n ₂ d ₂/cosθ₂ +n ₃ d ₃/cosθ₃ =nd/cosθ(Condition Equation b)d ₁tanθ₁ +d ₂tanθ₂ +d ₃tan θ₃ =dtanθ

These two condition equations a and b are simultaneously established andare solved with respect to n and d after being slightly changed by usingsinφ/sinφ₁=n₁, sinφ/sinφ₂=n₂, sinφ/sinφ₃=n₃ in accordance with theSnell's law and a condition equation c paraxially established, which isshown below, thereby obtaining the above-described combined refractiveindex n and combined thickness d.

(Condition Equation c)φ=0

While this embodiment has been described with respect to the case wherethe sheet layer 1 is a polycarbonate layer, the sheet layer 1 mayalternatively be a layer of an acrylic resin, a norbornene resin, anultraviolet curing resin or the like. Also, while the description hasbeen made of the case where the protective layer is constituted by threelayers, the same description applies to other cases where the protectivelayer is constituted by two layers or four or more layers.

In this embodiment of the present invention, −3 μm≦d−f(n)≦3 μm. However,setting −10 μm≦d−f(n)≦10 μm is effective in a case where recording orreproduction is performed with an optical head incorporating a sphericalaberration correction element FIG. 9 shows an example of use of aspherical aberration correction element. A spherical aberrationcorrection element 10 such as the one disclosed in Japanese PatentLaid-Open No. 2000-131603 is placed in the optical path of an opticalhead.

If variation in thickness of a disk is within the limits of 10 μm, adisk design margin is increased, the yield and productivity areimproved, and surface coating, in which limitation of thicknessvariation to a small value is difficult, is facilitated. If thethickness varies by 10 μm, a spherical aberration of about 100 mλrmsoccurs. However, such an aberration is corrected by the sphericalaberration correction element 10. For example, if variation in thicknessthrough one track on the disk is within the limits of 3 μm and ifvariation in thickness through the surface is within the limits of 10μm, a method of use is possible in which DC correction of a radialthickness error is made with the spherical aberration correctionelement.

(Embodiment 2)

Embodiment 2 of the present invention will be described with referenceto FIG. 18.

An optical disk 7 has a protective layer 4 and 4 d, two recording layers5 and 5 d, and a reinforcing substrate 6. Light having a wavelength of400 to 410 nm is converged by an objective lens 9, and the recordinglayers 5 and 5 d are irradiated with this light on the protective layer4 side, thereby performing recording and/or reproduction of information.An optical disk having two recording layers has been realized as a DVD.The amount of information can be approximately doubled thereby.

The numerical aperture of the objective lens 9 is large, about 0.85.Therefore two lenses are used as the objective lens 9. The objectivelens 9 is designed so that a third-order spherical aberration componentof wavefront aberration which occurs when light having a wavelength of405 nm passes through a light-transmitting flat plate made ofpolycarbonate or the like and having a refractive index of 1.62 and athickness of 100 μm is substantially zero. A spherical aberrationcorrection element 10 such as the one disclosed in Japanese PatentLaid-Open No. 2000-131603 is placed in the optical path of the opticalhead to correct a spherical aberration which occurs due to an error inthe thickness of the protective layers 4 and 4 d, which is 100 μm.

The recording layers 5 and 5 d are films containing a dielectric orreflecting films. The reinforcing substrate 6 prevents the optical disk7 from being warped or broken. The protective layer 4 protects therecording layer 5 against air. Also, a surface 8 of the optical disk 7is separated from the recording layer 5 by the protective layer 4 toprevent degradation of recording or reproduction performance due to duston the surface 8 or scratches in the surface 8.

The protective layer 4 related to the recording layer 5 is constitutedby a sheet layer 1 and a coating layer 2. The protective layer 4 drelated to the recording layer 5 d is constituted by the sheet layer 1,the coating layer 2, and an adhesive layer 3.

Polycarbonate or the like is used for the sheet layer 1, an acrylicresin or the like is used for the coating layer 2, and an ultravioletcuring resin or the like is used for the adhesive layer 3. Therefractive index of the sheet layer 1 at a wavelength of 405 nm is 1.62,the refractive index of the coating layer 2 at the same wavelength is1.50, and the refractive index of the adhesive layer 3 at the samewavelength is 1.53.

In the optical disk of the present invention, if the refractive indicesof the sheet layer 1, the coating layer 2 and the adhesive layer 3 at awavelength of 405 nm are n₁, n₂, and n₃, respectively; and the thicknessof these layers are d₁, d₂, and d₃ (μm) 1.45≦n₁≦1.65, 1.45≦n₂≦1.65,1.45≦n₃≦1.65, 1.45≦n₃1.65, and f(n)−10(μm)≦d≦g(n)+10 (μm) are satisfied.

With respect to the protective layer 4, the combined refractive index nand the combined thickness d can be expressed by the following equations4 and 5.

(Equation 4)n=((n ₁ d ₁ +n ₂ d ₂)/(d ₁ /n ₁ +d ₂ /n ₂))^(0.5)(Equation 5)d=((n ₁ d ₁ +n ₂ d ₂)×(d ₁ /n ₁ +d ₂ /n ₂))^(0.5)

With respect to the protective layer 4 d, the combined refractive indexn and the combined thickness d can be expressed by the followingequations 6 and 7.

(Equation 6)n=((n ₁ d ₁ +n ₂ d ₂ +n ₃ d ₃)/(d ₁ /n ₁ +d ₂ /n ₂ +d ₃ n ₃))^(0.5)(Equation 7)d=((n ₁ d ₁ +n ₂ d ₂ +n ₃ d ₃)×(d ₁ /n ₁ +d ₂ /n ₂ +d ₃ n ₃))^(0.5)

The combined refractive indices n and combined thickness d are acombined refractive index and a combined thickness specific to thepresent invention which are obtained by replacing the plurality oflayers with one layer equivalent to the plurality of layers. Aberrationof light passed through the plurality of layers and aberration of lightpassed through the one layer equivalent to the plurality of layers aresubstantially equal to each other.

Derivation of these equations 4 to 7 may be the same as the derivationmethod described in the description of Embodiment 1.

An expression f(n) and an expression g(n) shown as equations 8 and 9below are each a third-order approximate curve passing through discretepoints at which spherical aberration obtained when combined thicknesserrors of −15 μm and 15 μm are corrected by the spherical aberrationcorrection element is substantially zero in aberration calculation basedon ray tracing. The equations 8 and 9 are design criteria for thecombined thicknesses of the protective layers 4 and 4 d.

(Equation 8)f(n)=−105.8n ³+551.5n ²−936.9n+605.2(Equation 9)g(n)=−138.7n ³+723.7n ²−1228.7n+796.0

The combined refractive index n and the combined thickness d satisfyingeach of these conditions are in the region surrounded by curves andstraight lines in FIG. 3. Each of the dotted lines is the curve f(n) org(n) along which the spherical aberration is substantially zero as aresult of correction of thickness errors of ±15 μm by the sphericalaberration correction element.

If the combined refractive index n and the combined thickness d obtainedfrom the thicknesses and the refractive indices of the plurality oflayers constituting each protective layer are determined within thisregion (region surrounded by the solid lines in FIG. 3), an optical diskincorporating the spherical aberration correction element can befabricated in which the spherical aberration is limited substantially to30 mλrms.

If variation in thickness through one track on the disk is within thelimits of 3 μm and if radial thickness error is corrected by DCcorrection with the spherical aberration correction element, thespherical aberration is limited substantially to 30 mλrms. Correctionability of about ±20 μm in thickness can easily be realized as abilityof the spherical aberration correction element.

If the combined thickness of the layer between the first recording layerand the second recording layer is set to 20 μm or larger, this layerfunctions as a separation layer to separate focus error signals for thetwo recording layers. The effect of reducing the amount of stray lightcaused by refraction of the other recording layer can also be obtained.

Thus, in the embodiment of the present invention, the thickness andrefractive index of each of the plurality of layers constituting theprotective layer are considered to enable fabrication of an optical diskin which occurrence of spherical aberration is limited.

While the description has been made of the case where the sheet layer 1is a polycarbonate layer, the sheet layer 1 may alternatively be a layerof an acrylic resin, a norbornene resin, an ultraviolet curing resin orthe like. Also, while the description has been made of the case wherethe protective layer is constituted by three layers, the samedescription applies to other cases where the protective layer isconstituted by two layers or four or more layers.

(Embodiment 3)

Embodiment 3 of the present invention will be described with referenceto FIG. 9. An optical disk 7 of this embodiment has the sameconstruction as that in Embodiment 1 of the present invention.

In the optical disk 7 of the present invention, if the refractiveindices of the sheet layer 1, the coating layer 2 and the adhesive layer3 at a wavelength of 405 nm are n₁, n_(2a), and n_(2b), respectively;the thickness of the sheet layer 1 is d₁ (μm); and the thickness of theprotective layer 4 is d₁+d₂ (μm), 1.45≦n₁≦1.65, 1.50≦n_(2a)≦1.55,1.50≦n_(2b)≦1.55, and −3 μm≦d₁+d₂−f(n₁)≦3 μm are satisfied.

In this expression, f(n₁)=A·n₁ ³+B·n₁ ²+C·n₁+D, A=1.280d₂−109.8,B=−6.652d₂+577.2, C=11.27d₂−985.5, and D=−6.257d₂+648.6.

An expression f(n₁) is a third-order expression of n₁ using coefficientsA, B, C, and D each expressed by using d₂ as a parameter. Expressionf(n₁) expresses a third-order approximate curve passing through discretepoints at which spherical aberration is substantially zero in aberrationcalculation based on ray tracing. The thickness is prescribed within thelimits of 3 μm from this curve.

The refractive index n₁ and the thickness d₁+d₂ satisfying thiscondition are in the region surrounded by curves and straight lines inFIG. 4. The dotted line is the curve f(n₁) along which the sphericalaberration is substantially zero. This embodiment of the presentinvention is characterized in that the curve f(n₁) with the thickness d₂used as a parameter is changed by considering the influence of theadhesive layer 3 and the coating layer 2 on the spherical aberration. Ifthe refractive index n₁ and the thickness d₁+d₂ are determined withinthis region, an optical disk can be fabricated in which the sphericalaberration is limited substantially to 30 mλrms.

FIG. 5 shows the curve d₁+d₂=f(n₁) when the value of d₂ is changed.Also, points at which spherical aberration is substantially zero inaberration calculation based on ray tracing are plotted. Errorstherebetween are 0.1 μm or less and a good match can be recognizedtherebetween.

Thus, in the embodiment of the present invention, the thickness andrefractive index of each of the plurality of layers constituting theprotective layer are considered to enable fabrication of an optical diskin which occurrence of spherical aberration is limited.

While the description has been made of the case where the sheet layer 1is a polycarbonate layer, the sheet layer 1 may alternatively be a layerof an acrylic resin, a norbornene resin, an ultraviolet curing resin orthe like. Also, while the description has been made of the case wherethe protective layer is constituted by three layers, the samedescription applies to other cases where the protective layer isconstituted by two layers or four or more layers.

In this embodiment of the present invention, −3 μm≦d₁+d₂−f(n₁)≦3 μm.However, setting −10 μm≦d₁+d₂−f(n₁)≦10 μm is effective in a case whererecording or reproduction is performed with an optical headincorporating a spherical aberration correction element. FIG. 9 shows anexample of use of a spherical aberration correction element. A sphericalaberration correction element 10 such as the one disclosed in JapanesePatent Laid-Open No. 2000-131603 is placed in the optical path of anoptical head.

If variation in thickness of a disk is within the limits of 10 μm, adisk design margin is increased, the yield and productivity areimproved, and surface coating, in which limitation of thicknessvariation to a small value is difficult, is facilitated. If thethickness varies by 10 μm, a spherical aberration of about 100 mλrmsoccurs. However, such an aberration is corrected by the sphericalaberration correction element 10. For example, if variation in thicknessthrough one track on the disk is within the limits of 3 μm and ifvariation in thickness through the surface is within the limits of 10μm, a method of use is possible in which DC correction of a radialthickness error is made with the spherical aberration correctionelement.

(Embodiment 4)

Embodiment 4 of the present invention will be described with referenceto FIG. 9. An optical disk 7 of this embodiment has the sameconstruction as that in Embodiment 1 of the present invention.

In the optical disk 7 of the present invention, if the refractiveindices of the sheet layer 1, the coating layer 2 and the adhesive layer3 at a wavelength of 405 nm are n₁, n_(2a), and n_(2b), respectively;the thickness of the sheet layer 1 is d₁ (μm); and the thickness of theprotective layer 4 is d₁+d₂ (μm), 1.61≦n₁≦1.63, 1.50≦n_(2a)≦1.55,1.50≦n_(2b)≦1.55, and −3 μm≦d₂−f(d₁)≦3 μm are satisfied. Expressionf(d₁) is f(d₁)=0.986d₁+98.6, which is an approximate straight linepassing through discrete points at which spherical aberration issubstantially zero in aberration calculation based on ray tracing.

The thicknesses d₁ and d₂ satisfying this condition are in the hatchedregion in FIG. 6. If the thicknesses d₁ and d₂ are determined withinthis region, an optical disk can be fabricated in which the sphericalaberration is limited substantially to 30 mλrms.

This embodiment of the present invention is effective in the case wherepolycarbonate or the like is used for the sheet layer 1. While thedescription has been made of the case where the protective layer isconstituted by three layers, the same description applies to other caseswhere the protective layer is constituted by two layers or four or morelayers.

In this embodiment of the present invention, −3 μm≦d₂−f(d₁)≦3 μm.However, setting −10 μm≦d₂−f(d₁)≦10 μm is effective in a case whererecording or reproduction is performed with an optical headincorporating a spherical aberration correction element.

(Embodiment 5)

Embodiment 5 of the present invention will be described with referenceto FIG. 9. An optical disk 7 of this embodiment has the sameconstruction as that in Embodiment 1 of the present invention.

In the optical disk 7 of the present invention, if the refractiveindices of the sheet layer 1, the coating layer 2 and the adhesive layer3 at a wavelength of 405 nm are n₁, n_(2a), and n_(2b), respectively;the thickness of the sheet layer 1 is d₁ (μm); and the thickness of theprotective layer 4 is d₁+d₂ (μm), 1.49≦n₁≦1.51, 1.50≦n_(2a) ≦1.55, 1.50:n_(2b)≦1.55, and −3 μm≦d₂−f(d₁)≦3 μm are satisfied. Expression f(d₁) isf(d₁)=−1.002d₁+98.6, which is an approximate straight line passingthrough discrete points at which spherical aberration is substantiallyzero in aberration calculation based on ray tracing.

The thicknesses d₁ and d₂ satisfying this condition are in the hatchedregion in FIG. 7. If the thicknesses d₁ and d₂ are determined withinthis region, an optical disk can be fabricated in which the sphericalaberration is limited substantially to 30 mλrms.

The embodiment of the present invention is effective in the case wherean acrylic resin or the like is used for the sheet layer 1. While thedescription has been made of the case where the protective layer isconstituted by three layers, the same description applies to other caseswhere the protective layer is constituted by two layers or four or morelayers.

In this embodiment of the present invention, −3 μm≦d₂−f(d₁)≦3 μm.However, −10 μm≦d₂−f(d₁)≦10 μm is effective in a case where recording orreproduction is performed with an optical head incorporating a sphericalaberration correction element.

(Embodiment 6)

Embodiment 6 of the present invention will be described with referenceto FIG. 9. An optical disk 7 of this embodiment has the sameconstruction as that in Embodiment 1 of the present invention.

In the optical disk 7 of the present invention, if the refractiveindices of the sheet layer 1, the coating layer 2 and the adhesive layer3 at a wavelength of 405 nm are n₁, n_(2a), and n_(2b), respectively;the thickness of the sheet layer 1 is d₁ (μm); and the thickness of theprotective layer 4 is d₁+d₂ (μm), 1.52≦n₁≦1.54, 1.50≦n_(2a)≦1.55,1.50≦n_(2b)≦1.55, and −3 μm≦d₂−f(d₁)≦3 μm are satisfied.

Expression f(d₁) is f(d₁)=−d₁+98.6, which is an approximate straightline passing through discrete points at which spherical aberration issubstantially zero in aberration calculation based on ray tracing.

The thicknesses d₁ and d₂ satisfying this condition are in the hatchedregion in FIG. 8. If the thicknesses d₁ and d₂ are determined withinthis region, an optical disk can be fabricated in which the sphericalaberration is limited substantially to 30 mλrms.

The embodiment of the present invention is effective in the case where anorbornene resin or the like is used for the sheet layer 1. While thedescription has been made of the case where the protective layer isconstituted by three layers, the same description applies to other caseswhere the protective layer is constituted by two layers or four or morelayers.

In this embodiment of the present invention, −3 μm≦d₂−f(d₁)≦3 μm.However, setting −10 μm≦d₂−f(d₁)≦10 μm is effective in a case whererecording or reproduction is performed with an optical headincorporating a spherical aberration correction element.

Needless to say, the sheet layer 1 in Embodiments 1 to 6 of the presentinvention may be a layer of an ultraviolet curing resin.

(Embodiment 7)

FIG. 10 shows an example of the method of recording on or reproductionfrom the optical disk in accordance with one of Embodiments 1 to 6 byusing an optical head incorporating a spherical aberration correctionelement.

Approximately half of light emitted from a semiconductor laser 30 andhaving a wavelength of 405 nm passes through a prism 31 and iscollimated by the condenser lens 32 to become an approximately parallelbeam of light. The collimated beam is reflected by a mirror 33, passesthrough a spherical aberration correction element 34, is converged by anobjective lens 35 having a numerical aperture of 0.85, and forms a lightspot on the recording layer of an optical disk 37 of the presentinvention.

The light reflected by the recording layer again passes through theobjective lens 35 and the spherical aberration correction element 34, isreflected by the mirror 33, and is focused by the condenser lens 32.Approximately half of the focused light is reflected by the prism 31,passes through the cylindrical lens 38 and is detected by aphotodetector 39.

The photodetector 39 is arranged to detect a reproduction signal, afocus control signal for enabling the objective lens 35 to follow therecording layer of the optical disk 37 by an astigmatism method, and atracking control signal for enabling the objective lens 35 to follow atrack on the optical disk 37 by a phase difference method or a push-pullmethod. The objective lens 35 is driven in a focusing direction and in atracking direction by an objective lens driver 36 on the basis of thesecontrol signals.

The spherical aberration correction element 34 is, for example, one suchas disclosed in Japanese Patent Laid-Open No. 2000-131603. The sphericalaberration correction element 34 optimizes the light spot formed on therecording layer of the optical disk 37 by correcting sphericalaberrations which occur due to a thickness error and a refractive indexerror in the protective layer of the optical disk 37. Correction of aprotective layer thickness error of ±20 μm by the spherical aberrationcorrection element 34 can easily be realized. If variation in thicknessthrough one track on the disk is within the limits of 3 μm, theresulting spherical aberration can be limited to 30 mλrms by performingDC correction of the radial thickness error by the spherical aberrationcorrection element 34.

The spherical aberration correction element 34 is constituted by twolenses. The distance between the two lenses is changed in the directionof the optical axis to reduce the degree of divergence or convergence oflight incident on the objective lens 35, thereby correcting thespherical aberration.

The entire disclosure of Japanese Patent Laid-Open No. 2000-131603 areincorporated herein by reference in its entirety.

Thus, when recording on or reproduction from the optical disk of thepresent invention is performed with the optical head incorporating aspherical aberration correction element, the spherical aberration can belimited substantially to 30 mλrms, thereby realizing an improvement ininformation density as never before possible.

According to the present invention, design margins with respect to thethickness and refractive index of the optical disk are increased and theyield and productivity are improved.

Also, surface coating, in which limitation of thickness variation to asmall value is difficult, is facilitated.

Further, the effect of further increasing the density can be achieved byproviding two recording layers.

(Embodiment 8)

FIG. 11 is a diagram schematically showing an embodiment of the opticalinformation recording/reproduction apparatus of the present invention.

An optical disk 37 is the optical disk in accordance with one ofEmbodiments 1 to 6. The optical disk 37 is rotated by a motor 42. Anoptical head 40 is moved in a radial direction of the optical disk 37along a shaft 43.

For recording or reproduction of information, light emergent from asemiconductor laser of the optical head 40 is converged to the recordinglayer of the optical disk 37 by an objective lens 41. A photodetector ofthe optical head 40 detects a focus control signal for enabling theobjective lens 41 to follow the surface of the optical disk 37, and atracking control signal for enabling the objective lens 41 to follow aninformation track on the optical disk 37.

A head control circuit 44 performs focus control and tracking control onthe optical head 40 on the basis of these control signals.

A signal processing circuit 45 records information on the optical disk37 through the optical head 40 at the time of recording, and reproducesoptical information recorded on the information track on the opticaldisk 37 from a signal output from the photodetector of the optical head40 at the time of reproduction.

The optical information recording/reproduction apparatus of the presentinvention sufficiently reduces the spherical aberration in the spotformed on the recording layer of the optical disk 37 to achieve anincrease in information density.

A spherical aberration correction element such as the one disclosed inJapanese Patent Laid-Open No. 2000-131603 is incorporated in the opticalhead 40 to effectively reduce the spherical aberration even if theoptical disk has large variation in thickness of the protective layer orhas two recording layers, thus enabling normal information recording andreproduction.

The optical information recording medium of the present invention can beapplied in the same manner to either of a type capable of recording andreproduction of optical information and a reproduction-only type.

While the embodiments of the present invention have been described withrespect to the case where the predetermined wavelength is 405 nm, lighthaving a wavelength other than this, e.g., a wavelength in the rangefrom 400 to 410 nm, or any other wavelength may be used.

Also, while the embodiments have been described with respect to the casewhere the two recording layers are provided, the above-describedinvention can also be applied to the case where two or more recordinglayers are provided in a similar manner by modifying, according to thenumber of recording layers, the arrangement in the case where tworecording layers are provided.

Also, while the embodiments have been described with respect to the casewhere expression f(n), expression g(n) or the like representing anapproximate curve passing through points at which spherical aberrationis substantially zero is used as a design criterion relating to thecombined thickness of the protective layer, the coefficients, the order,etc. of the expression representing the approximate curve are notlimited to those described above. In short, an approximate curve orstraight line passing through points at which spherical aberration issubstantially zero in aberration calculation based on ray tracing maysuffice as a relational expression used as a design criterion. Thedegree of approximation etc. may be freely selected according topurposes.

As described above, there is provided a method of designing an opticalinformation recording medium having a recording layer and a protectivelayer including at least first to mth layers (m≧2) wherein if i is aninteger satisfying 1≦i≦m; the refractive index of the ith layer in theprotective layer at a predetermined wavelength is n_(i); and thethickness of the ith layer is d_(i), (a) a combined refractive index nof one layer substantially equivalent to the plurality of layersconstituting the protective layer and substituted for the plurality oflayers is specified as n=((n₁d₁+n₂d₂+ . . . +n_(m)d_(m))/(d₁/n₁+d₂/n₂+ .. . +d_(m)/n_(m)))^(0.5), (b) a combined thickness d of the one layer isspecified as d=((n₁d₁+n₂d₂+ . . . +n_(m)d_(m))×(d₁/n₁+d₂/n₂+ . . .+d_(m)/n_(m)))^(0.5), and (c) a relational expression f(n) representinga relationship between the combined refractive index n and the combinedthickness d such that spherical aberration is substantially zero isspecified as a design criterion relating to the combined thickness ofthe optical information recording medium, and wherein the refractiveindex n_(i) and the thickness d_(i) of each layer are determined on thebasis of the relational expression f(n). This optical informationrecording medium design method has the effects described below.

That is, according to the above-described design method, the combinationof the refractive index n_(i) and the thickness d_(i) of each layer caneasily be selected from a multiplicity of candidate combinations ofn_(i) and d_(i) satisfying the predetermined relational expressionprescribed as a design criterion as described above, or the like toreduce the initial residual spherical aberration substantially to zero.Therefore, even if variation in thickness of the protective layer to ±3μm is tolerated, it is possible to limit the spherical aberration in theoptical disk substantially to 30 mλrms.

Also, the range of selection of various parameters including therefractive index n_(i) and the thickness d_(i) of each layer is therebyextended to increase the degree of design freedom.

As described above, the present invention has the advantage of furtherreducing spherical aberration in an optical information recording mediumhaving a protective layer constituted by a plurality of layers incomparison with in the conventional art, and thereby enabling normalrecording and/or reproduction.

FIG. 2

-   #1 COMBINED THICKNESS-   #2 COMBINED REFRACTIVE INDEX-   #3 WAVELENGTH.    FIG. 3-   #1 COMBINED THICKNESS-   #2 COMBINED REFRACTIVE INDEX-   #3 WAVELENGTH.    FIG. 4-   #1 THICKNESS-   #2 REFRACTIVE INDEX-   #3 WAVELENGTH.    FIG. 5-   #1 THICKNESS-   #2 REFRACTIVE INDEX-   #3 VALUE COMPUTED BY RAY TRACING.    FIG. 6-   #1 THICKNESS-   #2 WAVELENGTH.    FIG. 7-   #1 THICKNESS-   #2 WAVELENGTH.    FIG. 8-   #1 THICKNESS-   #2 WAVELENGTH.    FIG. 11-   44 HEAD CONTROL CIRCUIT-   45 SIGNAL PROCESSING CIRCUIT.    FIG. 13-   #1 THICKNESS-   #2 REFRACTIVE INDEX.    FIG. 16-   #1 SPHERICAL ABERRATION-   #2 THICKNESS OF ADHESIVE LAYER-   #3 NUMERICAL APERTURE    -   Wavelength

1. An optical information recording medium comprising at least first andsecond recording layers, and a protective layer including a plurality oflayers, wherein when first to kth layers (k≧2) in the plurality oflayers in the protective layer between a surface of the opticalinformation recording medium and the first recording layer are specifiedas a first protective layer, and first to mth layers (m>k) in theplurality of layers in the protective layer between the surface of theoptical information recording medium and the second recording layer arespecified as a second protective layer, when i is an integer satisfying1≦i≦m; the refractive index of the ith layer in the protective layer ata wavelength of 405 nm is n_(i); and the thickness of the ith layer isd_(i), when (a-1) a combined refractive index n of one layer which issubstantially equivalent to the first protective layer and substitutedfor the first protective layer is specified as n=((n₁d₁+n₂d₂+ . . .+n_(k)d_(k))/(d₁/n₁+d₂/n₂+ . . . d_(k)/n_(k)))^(0.5); (b−1) a combinedthickness d of said one layer is specified as d=((n₁d₁+n₂d₂+ . . .+n_(k)d_(k))×(d₁/n₁+d₂/n₂+ . . . +d_(k)/n_(k)))^(0.5); and (c-1) anexpression f(n): f(n)=−105.8n³+551.5n²−936.9n+605.2 is specified as adesign criterion relating to the thickness of the first protectivelayers; and when (a-2) a combined refractive index n of one layer whichis substantially equivalent to the second protective layer andsubstituted for the second protective layer is specified asn=((n₁d₁+n₂d₂+ . . . +n_(m)d_(m))/(d₁/n₁+d₂/n₂+ . . .+d_(m)/n_(m)))^(0.5); (b-2) a combined thickness d of said one layer isspecified as d=((n₁d_(l)+n₂d₂+ . . . +n_(m)d_(m))×(d₁/n₁+d₂/n₂+ . . .+d_(m)/n_(m))) ^(0.5); and (c-2) an expression g(n):g(n)=−138.7n³+723.7n²−1228.7n+796.0 is specified as a design criterionrelating to the thickness of the second protective layers, therefractive index n_(i) of the ith layer and the combined refractiveindex n of the first and second protective layers is equal to or largerthan 1.45 and equal to or smaller than 1.65, a difference d−f(n) betweenthe combined thickness d of the first protective layer and the designcriterion expression f(n) is equal to or larger than −10 μm, and adifference d−g(n) between the combined thickness d of the secondprotective layer and the design criterion expression g(n) is equal to orsmaller than 10 μm.
 2. An optical information recording mediumcomprising a recording layer, and a protective layer including at leasttwo layers, wherein when a refractive index of a particular one of thelayers in the protective layer at a wavelength of 405 nm is n₁; athickness of the particular one of the layers is d₁ (μm); and athickness of the layers in the protective layer other than theparticular one layer is d₂ (μm), when (a) equations A, B, C, and Dincluding said d₂ are specified as A=1.280d₂−109.8, B=−6.652d₂+577.2,C=11.27d₂−985.5, and D=−6.257d₂+648.6, and (b) an expression f (n₁):f(n₁)=A·n₁ ³+B·n₁ ²+C·n₁+D is specified as a design criterion relatingto the thickness of the protective layer, the refractive index n_(i) isequal to or larger than 1.45 and equal to or smaller than 1.65, arefractive index of the layers in the protective layer other than saidparticular one layer at a wavelength of 405 nm is equal to or largerthan 1.50 and equal to or smaller than 1.55, and a value of d₁+d₂−f(n₁)is equal to or larger than −10 μm and equal to or smaller than 10 μm. 3.The optical information recording medium according to claim 2, whereinthe value of d₁+d₂−f(n₁) is equal to or larger than −3 μm and equal toor smaller than 3 μm.
 4. An optical information recording mediumcomprising a recording layer, and a protective layer including at leasttwo layers, wherein when a thickness of a particular one of the layersin the protective layer is d₁ (μm); a thickness of the layers in theprotective layer other than said particular one layer is d₂ (μm); and anequation f(d₁) including said d₁ is specified as f(d₁)=−0.986d₁+98.6, arefractive index of said particular one layer at a wavelength of 405 nmis equal to or larger than 1.61 and equal to or smaller than 1.63, arefractive index of the layers in the protective layer other than saidparticular one layer at the wavelength 405 nm is equal to or larger than1.50 and equal to or smaller than 1.55, and a value of d₂−f(d₁) is equalto or larger than −10 μm and equal to or smaller than 10 μm.
 5. Theoptical information recording medium according to claim 4, wherein thevalue of d₂−f(d₁) is equal to or larger than −3 μm and equal to orsmaller than 3 μm.
 6. An optical information recording medium comprisinga recording layer, and a protective layer including at least two layers,wherein when a thickness of a particular one of the layers in theprotective layer is d₁ (μm); a thickness of the layers in the protectivelayer other than said particular one layer is d₂ (μm); and an equationf(d₁) including said d₁ is specified as f(d₁)=−1.002d₁+98.6, arefractive index of said particular one layer at a wavelength of 405 nmis equal to or larger than 1.49 and equal to or smaller than 1.51, arefractive index of the layers in the protective layer other than saidparticular one layer at the wavelength 405 nm is equal to or larger than1.50 and equal to or smaller than 1.55, and a value of d₂−f(d₁) is equalto or larger than −10 μm and equal to or smaller than 10 μm.
 7. Theoptical information recording medium according to claim 6, wherein thevalue of d₂−f(d₁) is equal to or larger than −3 μm and equal to orsmaller than 3 μm.
 8. An optical information recording medium comprisinga recording layer, and a protective layer including at least two layers,wherein when a thickness of a particular one of the layers in theprotective layer is d₁(μm); a thickness of the layers in the protectivelayer other than said particular one layer is d₂ (μm); and an equationf(d₁) including said d₁is specified as f(d₁)=−d₁+98.6, a refractiveindex of said particular one layer at a wavelength of 405 nm is equal toor larger than 1.52 and equal to or smaller than 1.54, a refractiveindex of the layers in the protective layer other than said particularone layer at the wavelength 405 nm is equal to or larger than 1.50 andequal to or smaller than 1.55, and a value of d₂−f(d₁) is equal to orlarger than −10 μm and equal to or smaller than 10 μm.
 9. The opticalinformation recording medium according to claim 8, wherein the value ofd₂−f(d₁) is equal to or larger than −3 μm and equal to or smaller than 3μm.
 10. An optical information recording medium comprising a recordinglayer, and a protective layer including at least first to mth layers(m≧2), wherein when i is an integer satisfying 1≦i≦m; a refractive indexof the ith layer in the protective layer at a predetermined wavelengthis n_(i); and a thickness of the ith layer is d_(i), and when (a) acombined refractive index n of one layer which is substantiallyequivalent to the plurality of layers constituting the protective layerand substituted for the plurality of layers is specified asn=((n₁d₁+n₂d₂+ . . . +n_(m)d_(m))/(d₁/n₁+d₂/n₂+ . . .+d_(m)/n_(m)))^(0.5); (b) a combined thickness d of said one layer isspecified as d=((n₁d₁+n₂d₂+ . . . +n_(m)d_(m))×(d₁/n₁+d₂/n₂+ . . .+d_(m)/n_(m)))^(0.5); and (c) an expression f(n) which has the combinedrefractive index n as a variable, and which is one off(n)=−109.8n³+577.2n² −985.5n+648.6, f(n)=−105.8n³+551.5n²−936.9n+605.2,and f(n)=−138.7n³+723.7n²−1228.7n+796.0 is specified as a designcriterion relating to the combined thickness of the optical informationrecording medium, a difference between the combined thickness d and thedesign criterion expression f(n) is equal to or larger than −10 μm andequal to or smaller than 10 μm.
 11. An optical informationrecording/reproduction method comprising performing at least one ofrecording and reproduction of information on the optical informationrecording medium according to claims 2, 4, 6, 8, or 10 by using anoptical head having aberration correction means of correcting aberrationwhich occurs due to the thickness of the protective layer of the opticalinformation recording medium.
 12. An optical informationrecording/reproduction apparatus comprising: the optical informationrecording medium according to any one of claims 3 to 10; an opticalhead; a rotating unit which causes rotation of said optical informationrecording medium; a control unit which controls said optical head; andrecording/reproduction means of performing at least one of recording ofinformation on said optical information recording medium andreproduction of information from said optical information recordingmedium.
 13. The optical information recording/reproduction apparatusaccording to claim 12, wherein said optical head has aberrationcorrection means of correcting aberration which occurs due to athickness of the protective layer of said optical information recordingmedium.